Plasmid DNA modified by in vitro treatments was transformed in E. coli bacterial cells. A streptomycin-resistant strain, carrying the peculiar rpsL421 mutation, was used as a recipient for the cloning vector pNO1523, which carries the wild-type (streptomycin-sensitive) rpsL allele. Transformants were streptomycin-sensitive unless a change in plasmid sequence had occurred. The analysis of the MaeI restriction pattern of plasmids isolated from streptomycin-resistant transformants, together with the detection of the phenotype that they conferred to a streptomycin-dependent strain, allowed us to identify plasmids that had undergone recombination with the host chromosome. The number of these plasmids exceeded by far that of plasmids resulting from mutational events.